The present invention relates to the field of tires and, in particular, tires having reinforced sidewalls.
In general, a tire consists of several parts, each of which plays a precise role in the operation of the tire, namely a crown which is located radially towards the outside, that is to say radially towards the largest radii of the tire, and is generally reinforced by a belt reinforcement, it having a tread, said crown being extended radially towards the inside, that is to say radially towards the smallest radii, by two sidewalls which are themselves extended radially towards the inside by beads.
The role of the sidewalls is to transmit forces between the crown and the beads of the tire. For this, said sidewalls have at least one carcass reinforcement which can extend into the crown of the tire and which is furthermore securely anchored in the beads by suitable means.
The carcass reinforcement is formed of reinforcements connected to each other by an elastomeric material and generally, but not exclusively, made in the form of a ply.
The expression "securely anchored in the beads" means that the beads are designed in such as manner as to take up all of the forces which are developed in the carcass reinforcement under the effect of the inflation pressure and the effect of the rolling of the tire.
In the present document, the expression "assembly" designates any reinforcement formed by the combination of several elements of a rigidity far greater than the average rigidity of the elastomer mixes used in the tire industry, such as for instance cords, filaments or strands, whatever the material thereof.
In order to improve the high-speed performance of a tire and reduce as far as possible the amplitude of the vibrations of the sidewalls, and also in order to be able to transmit a large torque from the wheel to the ground by the tire, it is desirable to increase the circumferential rigidity of the sidewalls and, at the same time, to adjust this rigidity in the best way possible as a function of the radial position in question in said sidewall.
There are several patents which describe the placing of assemblies in circumferential direction in the sidewalls of a tire. Aside from the fact that these patents are all directed at solving manufacturing problems when using continuous circumferential reinforcements, no mention is made therein of the importance of effecting a suitable variation of the circumferential rigidity as a function of the radial position in the sidewalls.
In particular, French Patent 1 304 907 describes a process of manufacturing a tire each of the sidewalls of which is reinforced by a reinforcement arranged along the circumferential direction. Said process consists, first of all, of producing on a suitable form a ring formed by the spiral winding of a single reinforcement which is held in place by elastomeric material, and then placing said ring on a shaped tire blank in order to form one of the sidewalls of said tire.
Such a process, while it makes it possible in fact to manufacture tires having circumferential reinforcements, as assemblies for instance, is very expensive to carry out and furthermore does not make it possible easily to effect a suitable adjustment of the circumferential rigidity as a function of the radial position in the sidewall.
Another manner of procedure is described in French Patent 2 170 848. That patent describes the possibility, in the case of a manufacturing process comprising a shaping step, of manufacturing tires the sidewalls of which are reinforced in circumferential direction by continuous assemblies known as "breaking core assemblies"; these assemblies comprise a linear core cord around which the reinforcement assembly proper is helically wound. This assembly is placed by circumferential winding on the carcass reinforcement, which itself is arranged on a building drum before shaping. During the shaping of the carcass, said breaking-core assembly has to adopt a development which is all the greater the greater the shaping ratio. This is possible due to the structure of the reinforcement assembly; in fact, after an initial phase during which only the core of said assembly opposes the deformation, the core breaks as soon as the rupture force of said core is reached, then leaving the assembly free to lengthen practically without effort.
It may be recalled that the shaping ratio of a product entering into the composition of a tire is the ratio between the radius of the cylinder for the laying of this product upon manufacture and the value of the radius of said product after the shaping.
However, in this example, only the reinforcement elements arranged in the sidewalls towards the largest radii will have resumed a quasi-elongated shape, that is to say with a quasi-infinite helicoidal pitch. Therefore, the effect of reinforcement to traction is maximum only on the assemblies located furthest radially to the outside. It is impossible here also by this means to adjust the circumferential rigidity of the sidewall reinforcements as a function of the radial position in said sidewall.